The expected product lifetime of an electronic device (usually expressed in a number of years) is directly related to functionality requirements of the device. The functionality of devices can be various: MEMS devices typically have moving mechanical components and often additional circuitry, IR sensors have components that are sensitive to infra-red light, etc. Furthermore, for a large number of electronic components protection from corrosion, water vapor, and oxidation is required. In order to protect electronic device, prolongue the lifetime of electronic devices, and—for some applications—also provide appropriate conditions for allowing functioning, electronic devices typically may be packaged.
Packaging of electronic devices thus often is an essential process step in the production. For a large number of applications, it is important that the electronic devices comprise a package that is hermetically sealing components. The permeability of a material typically is defined as the rate at which gas atoms diffuse through a material. Permeation rates can be compared for materials of equal thickness and under standardized atmospheres. For example, by definition materials with less than 1 day of sealing capacity (10−14 gm/cm·s·Torr) are typically considered non-hermetic, a glass seal of 10 μm thick typically provides a seal lifetime of a few years, a metal seal of 10 μm thick typically provides a seal lifetime of a century or more.
Although considered “non-hermetic”, organic seals, such as epoxy seals, are known to be adequate to keep out moisture (water molecules), regardless of the thickness of the seal ring. Such sealing may be sufficient for consumer products such as image sensors, microfluidic devices and others that primarily require prevention of moisture penetration of the seal.
In other applications, however, such as e.g. many automotive applications, hermeticity is a requirement, also against moisture. It is known to provide metal seals for MEMS packaging in e.g. automotive applications, wherein the seals have a seal geometry of 1 to 2 μm which could conceivably be reduced to less than 1 μm if mechanical integrity (strength) could be ensured.
Hermetic sealing using e.g. a metal sealing ring can be performed accurately. A disadvantage of a metal sealing ring is that it typically may be subject to mechanical stress, which may lead to cracking, loss of the hermetic sealing, and finally failure of the device.
FIG. 1—prior art shows an example of a chip scale package for a micro component, as disclosed in WO2007/017757. The substrate 101 contains a sensor area 112, e.g. comprising a MEMS device. A cap structure 102 is attached to the sensor die by means of a seal ring 110 to hermetically encapsulate an area of the sensor die where the micro component is located, thus forming a hermetically sealed cavity 107. The seal ring may be a solder seal ring, which may be connected by thermo-compression bonding.
FIG. 2—prior art shows a so called “microcap package”, another specific type of packaging. A system is shown wherein the substrate 201 supports device 203, e.g. a MEMS device. A thick film or second substrate 202 is first aligned and then bonded to the first substrate 201 by means of a seal ring 210 to form a cap over device 201.